Method of hot-shaping and hardening a steel workpiece

ABSTRACT

A method of making a shaped and hardened steel part has the steps of sequentially heating a steel plate entirely above the AC 3  temperature, cooling specific regions of the heated plate without hardening the specific regions, and then engaging the heated plate with the cooled regions in a cool deforming tool with the specific regions out of contact with the deforming tool. This cools the plate where it contacts the tool and deforms the plate with the tool at the specific regions and both cooling and hardening the plate by contact with the tool.

FIELD OF THE INVENTION

The present invention relates to the shaping and hardening of steel. More particularly this invention concerns the manufacture of a shaped and hardened steel workpiece.

BACKGROUND OF THE INVENTION

It is standard practice to form a shaped and hardened steel workpiece, typically from a steel-alloy plate, by first heating a workpiece blank above the AC₃ point of the alloy, inserting it into a shaping tool in a press, shaping the hot blank, and quenching the shaped blank by contact with cool surfaces of the shaping tool.

Such a method is known from GB 1,490,535 for compression shaping and hardening of a thin steel sheet having good dimensional stability. The sheet is heated to a temperature above AC₃, typically 700° C. to 1000° C., and then, in less than 5 seconds, is pressed into the final shape between two indirectly cooled tools with significant deformation. The deformed sheet is then held in the press to rapid cool it and produce the desired martensitic and/or bainitic fine-grain structure. In practice, the described method has proven to have many uses in thermoforming of high-strength lightweight steel components, in particular for structural and safety-related parts in automotive-body manufacturing.

Depending on the complexity of the components to be manufactured, the components are preformed or, in a single work step, thermoshaped. For the deep-drawing process, also as a function of temperature of the workpiece blank, attention must be paid to the limits of deep-drawing. This is because, when the deformation limits of the material are reached, the material begins to get thinner and tear in the most severely stretched regions. DE 10 2004 038 626 describes the problem created when a workpiece in the shaping tool is pressed with very great force in some locations, and in other locations is hardly pressed at all. Between these two extremes, at various locations the sheet may also be clamped by forces that lie between the maximum force and a practically nonexistent force. As a result, unavoidable shrinkage of the component is prevented in regions where the component is strongly clamped, and more or less unpredictable shrinkage occurs in the regions where the clamping is less intense. This results in various material and shaped part characteristics, in particular various stress states or shrinkages. DE 10 2004 038 626 aims to remedy this problem by supporting the component by means of the quenching die in the vicinity of the positive radii, and by securely clamping some sections of the component without distortion, at least in the vicinity of the cut edges. In the regions where the component is not clamped, the component is separated by a gap from at least one half of a shaping tool, and the shape is adjusted and machined so that the component is able to freely undergo shrinkage outside the clamped regions, the component being pressed tightly against the mold, at least in the vicinity of the positive radii. Here shaping is expressly carried out in the cold state. Only shape deviations in the manner of hot calibration are corrected during the quenching process.

Regardless, working cold is not always desirable or possible. For example, steel coated with aluminum cannot be cold-formed without damaging the coating. Therefore, it is very important to thermoshape in one operation, even for complex three-dimensional shapes. Depending on the component shape, it may also be more cost-effective to thermoforming it from a blank in one pass, thereby avoiding multiple tool steps. If it is not possible to carry out the shaping in one step, it may be necessary to bring a component, already cold-preformed to a certain extent, to the final shape in the hot state in order to substitute at least one cold forming step. A blank as well as a component that is already preformed are consistently referred to in the following description as workpiece blanks, since the problem discussed below may occur for a blank as well as for a preformed component.

In the known thermoforming process, a workpiece blank made of steel is first heated to a temperature above the AC₃ point of the steel alloy. The heated workpiece blank is then inserted into an indirectly cooled shaping tool. The workpiece blank makes initial contact with the tool at the contact surface. Very often it is necessary, primarily for prevention of wrinkles, to deep-draw the parts using a blank holder. In particular for deep-drawing of workpiece blanks heated above 900° C., compared to drawing at low temperatures attention must be paid to circumstances that due to the temperature distribution in the workpiece blank during the shaping result in a reduction of the deep-drawing limits. During deep-drawing the heated workpiece blank first makes contact with the blank holder and the outermost points of the upper tool. As a result of this contact the workpiece blank undergoes intense cooling in these regions, and at these locations also has a much higher flow resistance. This effect is very disadvantageous for the drawing process, particularly in the vicinity of the blank holder from which the material is to continue to flow. For this reason the blank holders are also separated by spacers during thermoforming. In this manner a direct large-surface contact, and thus the undesired cooling of the workpiece blank in this region as a result of the tool, is avoided. At the same time, however, wrinkle formation is still suppressed.

The second special problem in deep-drawing of a hot workpiece blank is the large temperature difference between the regions of the workpiece blank that contact the upper tool or the bottom tool during shaping and the regions that have not yet contacted the tool. Since these regions are much weaker, but at the same time it is necessary to transmit the forces for drawing in the workpiece blank, this results in overstretching and thus tearing of the material. When the tool is closed, the hold-down element that is generally present thus makes early contact with the workpiece blank. The upper tool then meets the workpiece blank at a specified regions. The upper tool forces increasingly larger regions of the workpiece blank into the shape of the cavity until finally almost the entire surface area of the workpiece blank contacts the shaping tool in the closed press. On initial contact of the workpiece blank with the tool, the workpiece blank is cooled at the contact site; specifically, contact with the tool generally cools the workpiece blank so greatly that critical temperature gradients are created, resulting in higher strength in the contact regions. As a result, the regions of the workpiece blank that have not yet contacted the tool are hotter and weaker than the regions that are already solidifying. The workpiece blank therefore cools in a nonuniform manner during deep-drawing. Specifically the weak regions, which must absorb the deformation forces necessary for the deep-drawing, are then deep-drawn. In particular in the transition regions from the areas that are already relatively hard to areas with high deformation rates, the nonuniform cooling of the workpiece blank together with the mechanical strength values that are still nonuniformly distributed in the workpiece blank cause the workpiece blank to become thinner in the transition regions, and in the worst-case scenario, to tear. For this reason, limits are set for the deformation rates during thermoforming.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide an improved method of hot-shaping and hardening a steel workpiece.

Another object is the provision of such an improved method of hot-shaping and hardening a steel workpiece that overcomes the above-given disadvantages, in particular that extends these deep-drawing limits that are present during thermoforming in the prior art.

SUMMARY OF THE INVENTION

A method of making a shaped and hardened steel part. The method has according to the invention the steps of sequentially heating a steel plate entirely above the AC₃ temperature, cooling specific regions of the heated plate without hardening the specific regions, engaging the heated plate with the cooled regions in a cool deforming tool with the specific regions out of contact with the deforming tool and cooling the plate where it contacts the tool, and deforming the plate with the tool at the specific regions and both cooling and hardening the plate by contact with the tool.

In other words, according to the invention specific regions of the workpiece blank that must transmit deep-drawing forces during the shaping are precooled, after heating the entire workpiece blank to a temperature above AC₃ and before the regions are contacted by the shaping tool. This precooling is done without reaching the cooling rate necessary for hardening in the specific regions. The aim of the invention is to counteract the reduced shaping capability by localized cooling of the workpiece blank in these specific regions, before or during the shaping, thereby achieving a more uniform temperature distribution and elongation.

The workpiece blank is abruptly cooled as a result of its contact with the tool in the contact region and in the region of the hold-down element, and upon initial contact with the upper tool. This creates large temperature gradients inside the workpiece blank. These temperature gradients are reduced according to the invention as the result of proportional cooling of the specific regions that have not yet made contact. Ideally, the cooling causes the temperatures of the contacting and noncontacting regions to approach one another. Due to the proportional cooling, the strength in the yet unshaped regions is higher than if cooling is not performed. The yet unshaped regions are better able to transmit the forces necessary for deep-drawing, and can therefore undergo greater deformation without becoming thinner or tearing, as was the case heretofore. At the same time, the cooling of the yet uncontacted regions should not be so great that the regions harden before the shaping, since this would excessively increase resistance to plastic deformation. Austenite produced by heating the workpiece blank to a temperature above AC₃ is converted to martensite above a critical cooling rate. This critical cooling rate is achieved as a result of the contact with the tool, not by the blowing of gas against the workpiece to precool the specific regions. At the same time, for adequate hardening of the workpiece blank it is not necessary for the workpiece blank to already be cooled at the critical cooling rate down from the AC₃ point. Instead, sufficient martensite formation occurs even when the initial cooling temperature, from which cooling is performed at the critical cooling rate, is as much as several hundred degrees below the AC₃ point of the alloy. The invention makes use of this circumstance to extend the deep-drawing limits.

The temperature difference in the workpiece blank during closing motion of the shaping tool is reduced, and the potential deformation rates are increased. Notwithstanding, adequate martensite formation in the shaping tool during final shaping of the workpiece blank, and therefore sufficient hardening, occur in the previously uncooled regions as well as in the cooled regions.

Cooling of the regions of the workpiece blank that make later contact may occur before the workpiece blank is inserted into the shaping tool, or may occur in the shaping tool itself. Before insertion, the workpiece blank that has been heated above AC₃ may be locally cooled, for example in specific defined regions by contact with a cooling block. The workpiece blank may also be subjected to localized blowing in the defined regions. Alternatively, nozzles for blowing the workpiece blank for the purpose of precooling may be integrated into the press or into the shaping tool located in the press.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIGS. 1, 2, 3, and 4 are partly diagrammatic views illustrating steps of a shaping/hardening operation;

FIG. 5 is a diagram illustrating principles of this invention; and

FIG. 6 is another diagrammatic view illustrating another step of the method of this invention.

SPECIFIC DESCRIPTION

As seen in FIGS. 1-4 and 6, a flat steel plate constituting a workpiece blank 6 is formed into a cup 60.

FIG. 1 schematically illustrates parts of a deep-drawing die, specifically, an upper half or tool 1, a lower half or tool 2 having a cavity 3, a hold-down plate or element 4, and spacers 5. The workpiece blank 6 is a steel-alloy plate. All parts 1 through 6 are situated in a press (not illustrated in greater detail) that applies the shaping and locking forces. The heat distribution in the workpiece blank 6 is well described with reference to FIG. 1. Before being inserted into the deep-drawing tool, the blank 6 has been homogeneously heated to a temperature above the AC₃ point of the alloy. After the blank 6 is inserted into the deep-drawing tool, the blank 6 initially rests above the cavity 3, at the edge regions 7 and 8 on the lower tool 2. Immediately upon contact with the edge regions 7 and 8 the blank 6 rapidly cools, so that these regions 7 and 8 harden somewhat. The press is still open.

When the press is closed according to FIG. 2 the hold-down element 4 also contacts the blank 6 at the edge regions 7 and 8, which consequently become even colder and harder. The hold-down element 4 then rests on the spacers 5. In the illustrated embodiment of FIGS. 1 through 4, the upper tool 1 makes initial contact with the blank 6 in the center 9. Cooling also begins in this center 9 of the blank 6 as the result of contact with the upper tool 1. In the process stage shown in FIG. 2, the edge regions 7 and 8 and the center 9 of the blank 6 are then relatively cold and strong. In contrast, specific regions 10 and 11 of the blank that are not yet shaped have a relatively high temperature and are still relatively soft. Without of temperature compensation by cooling according to the invention, further closing of the upper tool 1 in the transition regions 12 and 13 would cause the blank 6 to gradually become thinner from the cold zone toward the hot zone, and also possibly tear.

According to the invention, this problem is counteracted by subjecting the blank 6 to localized cooling, either before being placed on the lower tool 2 or in the shaping tool in regions 10 and 11. As schematically illustrated in FIG. 6, cooling in the tool may be achieved, for example, by use of nozzles 17 and 18 that are integrated into the press. The cooling may also be performed by blowing or otherwise cooling outside the tool. However, the cooling must be limited so that hardening does not occur. The object is to achieve temperature balancing between the relatively cold regions 7, 8, and 9 and the regions 10 and 11 during the step FIG. 2 so that regions 10 and 11 can better transmit forces that are necessary for deep-drawing and that occur during the shaping process to the edge regions 7 and 8 of the blank 6, and so that the transition regions 12 and 13 from the already relatively strong zone to the still relatively weak zone inside the blank 6 are relieved of stress.

In FIG. 3 the upper tool 1 travels further down into the direction of the cavity 3. Deep-drawing forces are increasingly absorbed by the transition regions 12 and 13.

In FIG. 4 the press is completely closed and the cup 60 has been shaped from the blank 6. As a result of the complete tool contact the cup 60 is completely cooled and hardened within a holding time that depends on the sheet thickness and other factors. Contact with the tool as well as the holding time should be designed such that the workpiece is not only hardened, but also warping is minimized so that the workpiece satisfies tolerance requirements.

FIG. 5 shows flow curves 14, 15, 16 for the same steel material at different temperatures. Curve 14 shows the flow curve at a low steel temperature, curve 15 shows the flow curve at a moderate steel temperature, and curve 16 shows the flow curve at a high steel temperature. The curves are theoretical curves; the degree of plastic elongation that the material can actually withstand before failure is not illustrated. However, curves 14, 15, and 16 show that the colder the material (curve 14), the more tension must be applied to achieve the same elongation values as at higher temperatures (curve 16). This is because the flow resistance at high temperatures is lower than at low temperatures. For hot deep-drawing or thermoforming, these differing flow resistances mean that for temperature gradients in the workpiece blank the entire deep-drawing force applied as tension always results in a localized elongation in the hottest region of the workpiece blank, since the flow resistance is lowest at this localized point. The locally concentrated tension frequently results in the prior-art system with drastic temperature gradients in failure of the material during shaping. The invention counteracts this problem by equalizing the differing temperature levels, and therefore also equalizes the differing flow resistances by ensuring that the regions that must transmit the deep-drawing forces during the shaping process are cooled in advance. 

1. A method of making a shaped and hardened steel part, the method comprising the steps of sequentially: heating a steel plate entirely above the AC₃ temperature; cooling specific regions of the heated plate without hardening the specific regions; engaging the heated plate with the cooled regions in a cool deforming tool with the specific regions out of contact with the deforming tool and cooling the plate where it contacts the tool; and deforming the plate with the tool at the specific regions and both cooling and hardening the plate by contact with the tool.
 2. The shaping and hardening method defined in claim 1 wherein the specific regions are cooled by engagement with a cool block.
 3. The shaping and hardening method defined in claim 1 wherein the specific regions are cooled by blowing cool gas against them.
 4. The shaping and hardening method defined in claim 3 wherein the deforming tool is one half of a two-part mold and the specific regions are cooled before the other half of the mold is closed over the workpiece.
 5. A press for shaping and hardening a steel-plate workpiece that has been heated above the AC₃ temperature, the press comprising: a pair of cool mold halves closable on the workpiece, engageable with regions of the workpiece, and not engageable with specific other regions of the workpiece; and means for cooling the specific regions without hardening them prior to closing of the workpiece.
 6. The press defined in claim 5 wherein the means for cooling including cool-gas nozzles directed at the specific regions. 